Heterogeneous excess argon and Neoproterozoic heating in the Usagaran Orogen, Tanzania, revealed by single grain 40Ar/39Ar thermochronology
✍ Scribed by Steven M. Reddy; Alan S. Collins; Craig Buchan; Abdul H. Mruma
- Book ID
- 104019847
- Publisher
- Elsevier Science
- Year
- 2004
- Tongue
- English
- Weight
- 761 KB
- Volume
- 39
- Category
- Article
- ISSN
- 1464-343X
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✦ Synopsis
Existing tectonic models for the evolution of the Usagaran Orogen place much significance on Palaeoproterozoic K-Ar and Rb-Sr ages. Laser 40 Ar/ 39 Ar data from single mica grains from the Isimani Suite near the western margin of the orogen indicate that excess 40 Ar is common in micas and this casts considerable doubt on tectonic models that are based on previously published K-Ar ages. Biotites lying within a well-developed S 2 foliation (previously constrained at 1999-1991 Ma) yield ages up to 3.3 Ga and contain a significant excess 40 Ar component that is variable at an intra-and inter-sample scale. Textural evidence indicates that muscovite grew or recrystallized after the synkinematic biotites and they also record younger 40 Ar/ 39 Ar ages with individual steps from 524 to 1055 Ma. It is shown that the mica age variation does not reflect different periods of growth but the preferential partitioning of excess 40 Ar into the biotite. The muscovite data also have a component of excess 40 Ar and the youngest muscovite 40 Ar/ 39 Ar date (535.4 ± 2.3 Ma) indicates a maximum age for greenschist facies metamorphism. This date corresponds to thermal activity associated with the East African Orogen. Greenschist facies deformation (D 4 and D 5 ) is interpreted to have been coincident with this thermal event and indicates localized tectonic activity associated with Gondwanan amalgamation. The data are also consistent with greenschist facies deformation, metamorphism and deposition of the Usagaran Konse Group being of Neoproterozoic-Early Palaeozoic age. These new data therefore preclude a solely Palaeoproterozoic tectonic history for the Usagaran Orogen and indicate a complex thermal-tectonic reworking in the Neoproterozoic-Early Palaeozoic.